Add filter option to command

[Bladrak]

The option can be used as:

bin/php-plantumlwriter write /my/src/directory --filter="*.php" --filter="\!*Test.php" 

The escape '' is required for some shells

[davidfuhr]

This might resolve #3. But I do not like the filter inversion. How does it handle files actually starting with the exclamation mark (!)? I'd prefer something with separate options (like "--include" and "--exclude") similar to ant's fileset (https://ant.apache.org/manual/Types/fileset.html).